He made the discovery at Lowell Observatory in Flagstaff, Arizona,, during a systematic search for a trans-Neptunian planet (also called Planet X), which had been predicted by Percival Lowell and William Pickering. Pluto is named after the Roman god of the underworld, who was able to render himself invisible, and in honor of Lowell.
Tombaugh's discovery involved painstaking use of a blink comparator - a device which allows someone to compare two similar photographs by placing them in the viewer's field of vision, and then letting the user switch back and forth - blink - between the two. Tombaugh used the blink comparator to compare photographs of sections of sky taken months apart. A moving object, such as a planet, would appear to jump from one position to another while the more distant objects, such as stars, would appear stationary. Tombaugh noticed such a moving object in his search, and subsequent observations showed it to be the planet we call Pluto. The discovery was made on February 18, 1930, using images taken in January of the same year.
Tombaugh was born in Streator, Illinois. After his family moved to Burdett, Kansas, Tombaugh built his first telescope and sent drawings of his observations of Jupiter and Mars to the Lowell Observatory. These resulted in a job offer. Tombaugh was employed at the Lowell Observatory from 1929 to 1945. Following his discovery of Pluto, Tombaugh earned astronomy degrees from the University of Kansas and Northern Arizona University. He taught astronomy at New Mexico State University from 1955 until his retirement.